Question: On the first 4 quizzes of his history class, Michael got an average score of 78. What does he need on the next quiz to have an overall average of 81?
Answer: Let his score on the next quiz be $x$ The sum of all of his scores is then $4 \cdot 78 + x$ The same sum must also be equal to $5 \cdot 81$ Solve: $x = 5 \cdot 81 - 4 \cdot 78 = 93$.